1. Field of the Invention
This invention relates to an A/D converter for use with a delay circuit and more particularly to an A/D converter containing a variable integrator whose time constant is changed.
2. Description of the Prior Art
Hitherto, a surround sound of an audio machine has been produced by delaying a reproduced sound for a predetermined time, attenuating, and superposing the resulting sound. Such reproduction modes as a stadium mode and a church mode are available for delaying and superposing reproduced sounds by different techniques.
One of the delay circuits used for such purposes is shown in FIG. 1. In the circuit, an analog reproduction signal is temporarily converted into digital data by an A/D converter 10 and the digital data is stored in a memory 12. The data read from the memory 12 is converted back into analog data by a D/A converter 14. In the circuit, the write time into and read time out of the memory 12 are varied and the difference therebetween becomes a delay time.
Used as the A/D converter 10 is one as shown in FIG. 2. The A/D converter 10 comprises an adder 20, a quantizer 22, a variable integrator 24, and a control section 26; an analog output of the variable integrator 24 is fed back into the adder 20 to which an analog signal is input. The quantizer 22 comprises a comparator 22a which outputs high or low in response to input signal voltage and a latch circuit 22b which latches the output of the comparator 22a in response to a predetermined clock for converting an input signal into a 1-bit digital signal. The variable integrator 24 integrates outputs of the quantizer 22 for providing an analog signal corresponding to the input signal. The output of the variable integrator 24 is fed back into the adder 20, which then finds a difference between both the signals; for the input signal with no change, the quantizer 22 outputs high and low, namely, "1" and "0" alternately.
On the other hand, the control section 26 changes a time constant in the variable integrator 24 in response to the output state of the quantizer 22. That is, if the output level of the adder 20 is large and the quantizer 22 makes an unbalanced output between "0" and "1," the time constant of the variable integrator 24 is made small; if the output level of the adder 20 is small and the quantizer 22 outputs "0" and "1" keeping a balance therebetween, the time constant of the variable integrator 24 is made large. Under such control, when there is silence, the time constant can be made large to suppress occurrence of high-frequency noise; when the input signal changes, the time constant can be made small to output a high tone sufficiently.
In the D/A converter 14, the analog signal is restored in response to the signal read from the memory 12. At that time, the same variable integrator as the A/D converter 10 is used to provide the same analog signal as the input signal.
Thus, in the conventional circuit, the time constant of the variable integrator 24 is determined uniquely in response to the detected level. That is, the time constant is changed in the same degree for attack, in which sound becomes large rapidly, and for recovery, in which sound becomes small rapidly. If the signal subjected to such signal processing is reproduced, the listener feels a sense of acoustic disorder. If the time constant is set too large, high frequencies are cut too much in the beginning of attack; if the time constant is set too small, high frequencies are cut rapidly during recovery.
In the conventional device, the control width when the time constant is changed is always the same, and if the input signal changes rapidly and largely, it cannot be followed up and slant overload distortion on the waveform of reproduced sound becomes large.